This invention relates to rotating field machines in general and more particularly to a circuit for forming an electrical quantity which is proportional to a flux component in a rotating field machine.
Circuits of this general nature in which the rotating field machine has a first winding provided for the excitation and which comprise: an adder, to which a voltage proportional to the phase current in a lead of the machine, a voltage proportional to the differential quotient with respect to time of the phase current and a voltage proportional to the corresponding phase voltage are fed; an integrator which follows the adder and at the output of which the quantity proportional to the flux component is taken off; and a null regulator with PI characteristic which is designed for suppressing the d-c component of said quantity and the input of which is connected to the output of the integrator and the output of which is connected to a summing point at the input of the integrator are known. Such a circuit is shown in FIG. 4 of U.S. Pat. No. 3,593,083.
The known circuit forms the information regarding the position and magnitude of the flux vector in the rotating field machine directly from the terminal voltage and current. In a three phase machine, it is sufficient to measure the phase voltage and the phase current only twice. The circuit initially determines the main field voltage of the rotating field machine by subtracting the ohmic and the inductive voltage drop from the phase voltage, and subsequently determines the flux component by integration of the main field voltage. Two flux components determine the position of the flux vector and its magnitude.
Information regarding the position and magnitude of the flux vector makes it possible to operate the rotating field machine with field orientation (see Siemens-Zeitschrift 1971, pages 765 to 768 and U.S. Pat. No. 3,909,688).
When starting the rotating field machine, it is of particular importance to determine the flux components as accurately as possible. In the worst case, a false measurement can lead to an inability to start the rotating field machine.
From the circuit described in U.S. Pat. No. 3,593,083, it has been found that a sufficiently accurate determination of the position and magnitude of the flux vector, i.e., of the individual flux components, by direct measurement of the terminal voltage and current is possible only if, after the main switch and the excitation have been switched on, the rotating field machine has reached a speed which is a few percent, say, 5%, of the nominal speed. At lower speeds, the ohmic voltage drop, which is given by the product of the phase current and the ohmic resistance of the phase winding supplied and which is added in the adder is of the order of magnitude of the main field voltage to be determined, which causes considerable errors. Field oriented operation is therefore not possible during starting.